An electrically conductive contact between a component as referred to above, consisting of a more base metal, and a connecting element comprising a comparatively more noble metal cannot generally be satisfactorily avoided if substantial pressing or biasing forces are to be transmitted by means of the connecting element. At any event direct contact is an optimum consideration for applying and maintaining a biasing force. In contrast, an accumulation of electrolyte at that contact location between the connecting element and the component, which can occur solely due to moisture from the atmosphere, can be at least reduced or even prevented by using a protective ring.
A protective ring for resisting contact corrosion between a connecting element such as a screw which can be fitted to a component and which has a bearing surface, and the component itself, including a sealing ring of plastic material, is disclosed in DE 44 39 567. The protective ring is in the form of a cap consisting of plastic material, with and without an opening therethrough. If the protective ring does not have an opening therethrough, it can be fitted on to the head of the connecting element only subsequently after the connecting element has been fitted to the component to be secured thereby. If on the other hand the protective ring has an opening therethrough, it is possible to form a pre-assembled unit consisting of the connecting element and the protective ring, and that unit can then be employed for assembly purposes as it stands. The protective ring as indicated above comprises a sealing ring of plastic material and the sealing ring has as essential functional portions thereof a plurality of sealing lips which afford a sealing action in relation to the component on the one hand and in relation to the connecting element on the other hand. Such sealing lips are of a tapering cross-section and in the assembled condition of the protective ring bear more or less in linear contact against the component or the head of the connecting element respectively. The sealing lips co-operate with counterpart surfaces of a spherical or conical configuration, on the head of the connecting element. The connecting element used may be an adjusting screw. It is found that a substantial pressing force or even a biasing or prestressing force cannot be transmitted with that screw. In another design configuration of such a protective ring a part of the material of a sealing lip is clamped between the component and a bearing surface on the head of the connecting element, when the connecting element is fitted in position. That admittedly affords a good sealing action at that location in relation to contact corrosion. However, there is the disadvantage here that is not possible to prevent settlement losses and thus a drop in the biasing force originally applied by means of the connecting element, so that this connection is not always a sound one. Furthermore the use of sealing lips which are of a filigree-like structure can scarcely be reproducibly maintained precisely when dealing with loose items, as screws, nuts or the like actually are. Due to mutual contact between the loose articles there is the danger of the sealing lips suffering local damage before the connecting element in question is fitted in position. However, even if the sealing lips remain undamaged, it must be expected that the corresponding co-operation surface on the component may be in part rough and/or scored, in which case such inevitable unevenness in the co-operating surface of the component cannot be compensated for by the sealing lip. A sound reproducible sealing effect does not seem to be a viable option with that protective ring.
The journal METALL, issue 6, 1992, page 570, `Korroslonseigenschaften von nauen Magnesiumlogierungen`, A. L. OLSEN discloses a screw design for the prevention of contact corrosion, using a cap of plastic material which extends in the axial direction over the height of the head of a screw with a hexagonal recess in its head, the cap engaging with a radially inwardly directed flange into a space extending in an annular configuration around a separately provided cylindrical projection portion on the head of the screw adjoining the bearing surface thereof. That thereof involves using a plastic cover means in order to minimize the free or exposed surface area of the screw head.